High sensitivity and amenability to miniaturization for field-portable applications have helped to make ion mobility spectrometry (IMS) an important technique for the detection of many compounds, including narcotics, explosives, and chemical warfare agents as described, for example, by G. Eiceman and Z. Karpas in their book entitled “Ion Mobility Spectrometry” (CRC, Boca Raton, 1994). In IMS, gas-phase ion mobilities are determined using a drift tube with a constant electric field. Ions are separated in the drift tube on the basis of differences in their drift velocities. The drift velocity of an ion is proportional to the applied electric field strength at low electric field strength, for example 200 V/cm, and the mobility, K, which is determined from experimentation, is independent of the applied electric field. Additionally, in IMS the ions travel through a bath gas that is at sufficiently high pressure that the ions rapidly reach constant velocity when driven by the force of an electric field that is constant both in time and location. This is to be clearly distinguished from those techniques, most of which are related to mass spectrometry, in which the gas pressure is sufficiently low that, if under the influence of a constant electric field, the ions continue to accelerate.
E. A. Mason and E. W. McDaniel in their book entitled “Transport Properties of Ions in Gases” (Wiley, N.Y., 1988) teach that at high electric field strength, for instance fields stronger than approximately 5,000 V/cm, the ion drift velocity is no longer directly proportional to the applied electric field, and K is better represented by Kh, a non-constant high field mobility term. The dependence of Kh on the applied electric field has been the basis for the development of high field asymmetric waveform ion mobility spectrometry (FAIMS). Ions are separated in FAIMS on the basis of a difference in the mobility of an ion at high field strength, Kh, relative to the mobility of the ion at low field strength, K. In other words, the ions are separated due to the compound dependent behavior of Kh, as a function of the applied electric field strength.
In general, a device for separating ions according to the FAIMS principle has an analyzer region that is defined by a space between first and second spaced-apart electrodes. Often, the first electrode is maintained at ground potential while the second electrode has an asymmetric waveform V(t) applied to it. The asymmetric waveform V(t) is composed of a repeating pattern including a high voltage component, Vh, lasting for a short period of time th and a lower voltage component, Vl, of opposite polarity, lasting a longer period of time tl. The waveform is synthesized such that the integrated voltage-time product, and thus the field-time product, applied to the second electrode during each complete cycle of the waveform is zero, for instance Vhth+Vltl=0; for example +2000 V for 10 μs followed by −1000 V for 20 μs. The peak voltage during the shorter, high voltage portion of the waveform is called the “dispersion voltage” or DV.
Generally, the ions that are to be separated are entrained in a stream of gas flowing through the FAIMS analyzer region, for example between a pair of horizontally oriented, spaced-apart electrodes. Accordingly, the net motion of an ion within the analyzer region is the sum of a horizontal x-axis component due to the stream of gas and a transverse y-axis component due to the applied electric field. During the high voltage portion of the waveform an ion moves with a y-axis velocity component given by vh=KhEh, where Eh is the applied field, and Kh is the high field ion mobility under operating electric field, pressure and temperature conditions. The distance traveled by the ion during the high voltage portion of the waveform is given by dh=vhth=KhEhth, where th is the time period of the applied high voltage. During the longer duration, opposite polarity, low voltage portion of the asymmetric waveform, the y-axis velocity component of the ion is vl=KEl, where K is the low field ion mobility under ambient pressure and temperature conditions. The distance traveled is dl=vltl=KEltl. Since the asymmetric waveform ensures that (Vhth)+(Vltl)=0, the field-time products Ehth and Eltl are equal in magnitude. Thus, if Kh and K are identical, dh and dl are equal, and the ion is returned to its original position along the y-axis during the negative cycle of the waveform. If at Eh the mobility Kh>K, the ion experiences a net displacement from its original position relative to the y-axis. For example, if a positive ion travels farther during the positive portion of the waveform, for instance dh>dl, then the ion migrates away from the second electrode and eventually will be neutralized at the first electrode.
In order to reverse the transverse drift of the positive ion in the above example, a constant negative dc voltage called the “compensation voltage” or CV can be applied to the second electrode. This dc voltage prevents the ion from migrating toward either the second or the first electrode. If ions derived from two compounds respond differently to the applied high strength electric fields, the ratio of Kh to K may be different for each compound. Consequently, the magnitude of the CV that is necessary to prevent the drift of the ion toward either electrode is also different for each compound. Thus, when a mixture including several species of ions, each with a unique Kh/K ratio, is being analyzed by FAIMS, only one species of ion is selectively transmitted to a detector for a given combination of CV and DV. In one type of FAIMS experiment, the applied CV is scanned with time, for instance the CV is slowly ramped or optionally the CV is stepped from one voltage to a next voltage, and a resulting intensity of transmitted ions is measured. In this way a CV spectrum showing the total ion current as a function of CV, is obtained.
U.S. Pat. No. 5,420,424, issued to Carnahan and Tarassov on May 30, 1995, teaches a FAIMS device having cylindrical electrode geometry and electrometric ion detection, the contents of which are incorporated herein by reference. The FAIMS analyzer region is defined by an annular space between inner and outer cylindrical electrodes. In use, ions that are to be separated are entrained into a flow of a carrier gas and are carried into the analyzer region via an ion inlet orifice. Once inside the analyzer region, the ions become distributed all the way around the inner electrode as a result of the carrier gas flow and ion-ion repulsive forces. The ions are selectively transmitted within the analyzer region to an ion extraction region at an end of the analyzer region opposite the ion inlet end. In particular, a plurality of ion outlet orifices is provided around the circumference of the outer electrode for extracting the selectively transmitted ions from the ion extraction region for electrometric detection. Of course, the electrometric detectors provide a signal that is indicative of the total ion current arriving at the detector. Accordingly, the CV spectrum that is obtained using the Carnahan device does not include information relating to an identity of the selectively transmitted ions. It is a limitation of the Carnahan device that the peaks in the CV spectrum are highly susceptible to being assigned incorrectly. It is another limitation of the Carnahan device that the ions are consumed upon being detected at the electrometric detector. Accordingly, it is not possible to perform further analysis or separation of the ions, or to collect the ions for other uses.
Replacing the electrometric detector with a mass spectrometer detection system provides an opportunity to obtain additional experimental data relating to the identity of ions giving rise to the peaks in a CV spectrum. For instance, the mass-to-charge (m/z) ratio of ions that are selectively transmitted through the FAIMS at a particular combination of CV and DV can be measured. Additionally, replacing the mass spectrometer with a tandem mass spectrometer makes it possible to perform a full-fledged structural investigation of the selectively transmitted ions. Unfortunately, the selectively transmitted ions are difficult to extract from the analyzer region of the Carnahan device for subsequent detection by a mass spectrometer. In particular, the orifice plate of a mass spectrometer typically includes a single small sampling orifice for receiving ions for introduction into the mass spectrometer. This restriction is due to the fact that a mass spectrometer operates at a much lower pressure than the FAIMS analyzer. In general, the size of the sampling orifice into the mass spectrometer is limited by the efficiency of the mass spectrometer vacuum system. In principle, it is possible to align the sampling orifice of a mass spectrometer with a single opening in the FAIMS outer electrode of the Carnahan device; however, such a combination suffers from very low ion transmission efficiency and therefore poor detection limits. In particular, the Carnahan device does not allow the selectively transmitted ions to be concentrated for extraction through the single opening. Accordingly, only a small fraction of the selectively transmitted ions are extracted from the analyzer region, the vast majority of the selectively transmitted ions being neutralized eventually upon impact with an electrode surface.
Guevremont et al. describe the use of curved electrode bodies, for instance inner and outer cylindrical electrodes, for producing a two-dimensional atmospheric pressure ion focusing effect that results in higher ion transmission efficiencies than can be obtained using, for example, a FAIMS device having parallel plate electrodes. In particular, with the application of an appropriate combination of DV and CV an ion of interest is focused into a band-like region between the cylindrical electrodes as a result of the electric fields which change with radial distance. Focusing the ions of interest has the effect of reducing the number of ions of interest that are lost as a result of the ion suffering a collision with one of the inner and outer electrodes.
In WO 00/08455, the contents of which are incorporated herein by reference, Guevremont and Purves describe an improved tandem FAIMS/MS device, including a domed-FAIMS analyzer. In particular, the domed-FAIMS analyzer includes a cylindrical inner electrode having a curved surface terminus proximate the ion outlet orifice of the FAIMS analyzer region. The curved surface terminus is substantially continuous with the cylindrical shape of the inner electrode and is aligned co-axially with the ion outlet orifice. During use, the application of an asymmetric waveform to the inner electrode results in the normal ion-focusing behavior as described above, and in addition the ion-focusing action extends around the generally spherically shaped terminus of the inner electrode. This causes the selectively transmitted ions to be directed generally radially inwardly within the region that is proximate the terminus of the inner electrode. Several contradictory forces are acting on the ions in this region near the terminus of the inner electrode. The force of the carrier gas flow tends to influence the ions to travel towards the ion-outlet orifice, which advantageously also prevents the ions from migrating in a reverse direction, back towards the ion source. Additionally, the ions that get too close to the inner electrode are pushed back away from the inner electrode, and those near the outer electrode migrate back towards the inner electrode, due to the focusing action of the applied electric fields. When all forces acting upon the ions are balanced, the ions are effectively captured in every direction, either by forces of the flowing gas, or by the focusing effect of the electric fields of the FAIMS mechanism. This is an example of a three-dimensional atmospheric pressure ion trap, as described in greater detail by Guevremont and Purves in WO 00/08457, the contents of which are incorporated herein by reference.
Guevremont and Purves further disclose a near-trapping mode of operation for the above-mentioned tandem FAIMS/MS device, which achieves ion transmission from the domed-FAIMS to a mass spectrometer with high efficiency. Under near-trapping conditions, the ions that accumulate in the three-dimensional region of space near the spherical terminus of the inner electrode are caused to leak from this region, being pulled by a flow of gas towards the ion-outlet orifice. The ions that are extracted from this region do so as a narrow, approximately collimated beam, which is pulled by the gas flow through the ion-outlet orifice and into a smaller orifice leading into the vacuum system of the mass spectrometer. Accordingly, such tandem FAIMS/MS devices are highly sensitive instruments that are capable of detecting and identifying ions of interest at part-per-billion levels.
Unfortunately, the tandem FAIMS/MS arrangement suffers from a number of limitations. In particular, ions that are analyzed by mass spectrometry cannot be collected or analyzed further. Instead, the ions are neutralized upon impact with a detector element of the mass spectrometer, such as for instance an electron multiplier. Accordingly, it is not possible to analyze ions that are selectively transmitted by a first FAIMS device before they are provided to a second FAIMS device for additional separation in a tandem FAIMS/FAIMS arrangement. Similarly, it is not possible to provide the mass analyzed ions to a second detector for subsequent analysis by a complementary technique. Of course, analysis by a complementary technique provides an opportunity to probe characteristics of the ions other than mass-to-charge (m/z) ratio. For example, using an infrared analyzer to obtain the infrared spectrum of the ions provides information relating to the presence of specific chemical functional groups, etc.
Furthermore, the size of the sampling orifice into the mass spectrometer is very small, being limited by the efficiency of the mass spectrometer vacuum system. In order to transmit as many ions as possible from the FAIMS analyzer to the mass spectrometer, it is necessary to dispose the sampling orifice immediately adjacent to the ion-outlet orifice, such that widening of the ion beam as a result of ion diffusion and ion-ion repulsion is minimized. As will be obvious to one of skill in the art, the insertion of a non-destructive analyzer, such as for instance the above-mentioned infrared analyzer, intermediate the sampling orifice and the ion-outlet orifice results in a longer ion path to the mass spectrometer, which increases the amount of time for the ion beam to spread out radially. Of course, the efficiency of introducing ions into the mass spectrometer decreases as the cross section of the ion beam increases, and dilute samples may produce insufficient signal intensity for obtaining meaningful results.
It would be advantageous to provide a FAIMS apparatus including a detection system that overcomes the limitations of the prior art.